The invention relates to a system and a method for applying an intensity-modulated proton therapy on a predetermined tumor volume within the body of the patient.
The number of new hospital-based facilities for charged particles therapy is growing quickly, especially in Japan and in the USA. Proton therapy is becoming a realistic therapeutic possibility for treating well-selected cancer types in centralized large hospitals.
All new commercial systems are based on the passive scattering technique. The compact gantry for proton therapy at the Swiss Paul Scherrer Institute (PSI) is still the only facility in the world using a dynamic beam delivery technique based on the active scanning of a small proton pencil beam. Only at the German GSI facility (GSI=Gesellschaft fur Schwerionenforschung, Darmstadt) is a similar beam delivery technique used, with carbon ions in a horizontal beam line. By using this modern approach to radiation therapy the conformal shaping of the dose is achieved just by computer control without the need of field specific hardware.
This approach to proton therapy is attracting more and more the interest from industry and hospitals internationally, because this method is now being recognized as the preferred method for providing intensity-modulated radiation therapy (IMRT) using proton beams, a technique now known in the community as intensity-modulated-proton therapy (IMPT). This technique (based on the modulation of the totally delivered beam fluence) should not be confused with the active dynamic control of the beam intensity (the instantaneous dose rate) described later below. In contrast to photon-IMRT, with proton beam scanning one can control independently the flux, the range, the position and direction of each proton pencil beam, making use of all available degrees of freedom including the depth of penetration of the proton beam into the patient. With this method similar results can be obtained as with IMRT but with improved conformity and with a significant reduction (by a factor of two or three) of the dose burden on the healthy tissues surrounding the tumor.
The practical feasibility of IMPT was demonstrated recently at PSI by applying this new technique to a few of the clinical cases treated on the PSI Gantry 1.
Subsequently, the main features of the existing PSI Gantry 1 are described to recall the strengths and the weak points of the present system.
The PSI Gantry 1 is operational since 1997. By the end of 2003 166 patients have been treated with this system, with tumors located mainly in the skull, spinal cord and in the low pelvis. Currently up to 17 patients are treated per day. The preliminary clinical results obtained by using the new beam scanning technique are very encouraging.
The positive aspects achieved with the present system, PSI Gantry 1, are:                a) the possibility to apply conformal therapy with variable modulation of the range delivered “all by computer” without the need to use field specific beam shaping devices like collimators and compensators;        b) the capability to apply multiple fields without the personnel entering the treatment room; and        c) the capability to deliver IMPT (presently only on well immobilized targets).        
The concepts for this system are now more than 13 years old, the system was built with limited resources, only for the goal of showing the feasibility of scanning. For a better understanding of the technical improvements introduced by the inventions for the PSI Gantry 2 the main technical features of the PSI Gantry 1 are briefly given below:
The patient table is mounted eccentrically on the PSI Gantry 1 and rotates with the gantry. A counter-rotation maintains the patient table horizontal at any time. This choice was dictated by the limited amount of space available for the gantry. Without pushing the compactness of the system down to only 4 m diameter it would not have been possible to build such a Gantry 1 system at PSI. The eccentric mounting of the patient is now the most often criticized point, since the patient table moves away from the floor and the patient couch is not accessible to the personnel when the proton beam is applied from below.
The beam delivery used on Gantry 1 is a “discrete” spot scanning method (a “step-and-shoot” method), based on maximum simplicity. A proton pencil beam with a Gaussian profile of 7-8 mm FWHM is used and the beam is scanned in steps of 5 mm. The beam flux is measured with parallel plate ionization monitors in front of the patient. The beam is switched off during the displacement of the beam in-between spots with a kicker magnet. The most often used scan motion is done with a sweeper magnet installed upstream of the last 90° bending magnet. The beam optics is designed such that the action of the sweeper results in a parallel displacement of the swept beam in the patient. The magnetic scanning is applied on Gantry 1 only in one lateral direction, in the dispersive plane of the gantry (the direction U along the gantry axis). A range shifter system is used in front of the patient for providing fast changes of the penetration depth of the beam (second direction S of scanning, in depth). The range shifter consists of a stack of 5 mm-thick polyethylene plates, which are moved sequentially into the beam by pneumatic valves. The motion of the patient table is used as the third axis of scanning. This is the slowest and most seldom used motion, which is applied in the transverse non-dispersive direction T. The whole is a Cartesian beam scanning system.
The points to be improved on an inventive system and an inventive method in comparison to the present system are: The unsatisfactory access to the patient table when the beam is applied from below. The slow speed of scanning, due to the chosen sequence of the scanning devices, magnetic 1st, range 2nd and table 3rd. The use of two mechanical systems makes the scan too slow for applying repeatedly target repainting. The scanning on Gantry 1 is therefore quite sensitive to organ movements.
Further, a system for an intensity-modulated proton therapy of a predetermined volume within an object is known in the prior art, comprising:                a) a proton source in order to generate a proton beam being adjustable with respect to the beam intensity;        b) a degrader being optionally disposed in the proton beam in order to attenuate the energy of the protons in the proton beam to a desired proton energy in the proton beam;        c) a number of proton beam bending and/or focusing units constituting the beam line for the transport of the beam, where a section of the beam line can be rotated around the patient table (the actual proton gantry);        d) a beam nozzle on the gantry having an outlet for the proton beam to penetrate the predetermined volume of the object;        e) a beam bending magnet being disposed upstream of the nozzle; and        f) a couple of sweeper magnets being disposed upstream of said beam bending magnet in order to sweep in a parallel fashion the proton beam in both lateral direction at the exit of the last beam bending magnet.        
The method to deliver the dose by a double magnetic scanning and by changing the beam energy is known.